1) Field of the Invention
The present invention relates to a fuel cell system comprising a fuel cell made to generate electric energy through electrochemical reactions between hydrogen and oxygen, and effectively applicable to generators for movable bodies such as vehicles and ships, portable generators, or home-service generators.
2) Description of the Related Art
In a fuel cell system comprising a fuel cell stack made to generate electric energy through electrochemical reactions between hydrogen and oxygen, lack of the water content of a solid electrolyte membrane or film causes a drop of the electric conductivity of the electrolyte to increase the resistance of the electrolyte, which leads to a decrease in cell power.
In addition, in a case in which excessive water exists on electrode portions of a fuel electrode or an air electrode, the inhibition of the electrochemical reaction at the electrode surfaces occurs to decrease the cell power.
In particular, in the case of the fuel cell system, for the purpose of enhancing the fuel utilization factor, in most cases, the hydrogen unused in a fuel cell is re-supplied, or the outlet side of a fuel passage is closed to prevent the emission of hydrogen. On the other hand, in the system operation, the outlet side of an air passage is generally placed into an open condition. Accordingly, water diffuses from the air electrode side through the electrolyte membrane to the fuel electrode side, and the diffused water tends to accumulate so that the water may exist excessively on the fuel electrode side.
Although the water accumulation or residence around the fuel electrode is preventable in a manner such that the fuel electrode or a valve, or the like, provided in the fuel passage is opened to purge the water therefrom and, hence, a drop of the power is avoidable, hydrogen is emitted into the atmosphere during the purging, which causes lowering the fuel efficiency and the safety.
From these points of view, there is a need to prevent the water residence at the electrode portions while properly maintaining the water content of the electrolyte membrane.
Meanwhile, Japanese Laid-Open No. HEI 11-191423 discloses a fuel cell system in which the water supply quantity to an oxidizer gas and a fuel gas is controlled in accordance with a property of a fuel cell to adjust the water content of an electrolyte membrane. There is a problem which arises with the system disclosed in the foregoing document, which is designed to merely control the water supply quantity to the oxidizer gas and the fuel gas, however, in that difficulty is encountered in preventing the water from becoming excessive at the electrode portions while maintaining a proper water content of the electrolyte membrane, or in that difficulty is experienced in humidifying the electrolyte membrane quickly.